Transformers are used for galvanic isolation between an input and an output, and/or to ‘transform’ the impedance; i.e., the ratio of voltage to current at a given power level. Such transformers typically consist of at least two coupled windings on a common ferromagnetic core, a nominal “primary” winding to which input power is conventionally applied, and a “secondary” winding which provides the output power.
Transformer Core Materials
Various transformer core materials and configurations are known in the art. These materials include silicon-steel (Si-steel) in laminated or tape wound form, ferrite, and amorphous and nanocrystalline alloys (in tape wound form), with benefits and drawbacks to each of these materials in various applications. The present invention applies to high leakage inductance transformers with tape wound cores.
The distinction between core laminations and tape (also called “ribbon”) is largely based on thickness and the method of assembly. Core laminations are relatively thick, typically greater than 0.1 mm, and are stacked or assembled flat. Core tape materials are generally somewhat thinner than 0.1 mm, and are typically wound around a suitable form or mandrel to provide the desired shape.
Tape wound cores may be used in the “as wound” state, but are often cut into two pieces (cut cores) for assembly with windings. “Bars” (or “bricks”) may also be cut from sections of a wound core, and core assemblies may be made from some combination of bars and/or cut cores.
Comparison of Ferrite and Nanocrystalline Tape Cores
Ferrite is a well-known transformer core material and has been one of the principal core materials of choice for frequencies above about 5 to 10 kHz due to low hysteressis and eddy current losses. Although amorphous cores have a somewhat higher saturation flux density, modern nanocrystalline materials have lower hysteressis losses, lower than ferrites up to about 200 kHz and can still operate with 1.6 times the ac flux at 40 kHz and twice the ac flux at 20 kHz for the same loss (based on published data). Furthermore, the nanocrystalline material's saturation flux density BSAT is about 3 times that of ferrites at elevated temperatures of 80-100 degrees C. (1.2 Tesla v. 400 mT). Other tape wound materials with superior properties may yet be developed.
A drawback to nanocrystalline (and other tape wound and laminated core) materials is that the losses are low only when flux flows along the direction of the tape surface; any significant flux which flows normal to the tape surface (e.g., between tape layers, or into the external broad surface of the tape) creates large eddy current losses in the core. Ferrite, on the other hand, has the advantage of being an isotropic ceramic material, allowing flux to flow in any direction in the core without excess losses. (Various “distributed gap” core materials, such as powdered iron, also have the isotropic advantages of ferrite, but their permeabilities are generally too low for most transformer applications.)
Transformer Leakage Inductance
All transformers have a finite leakage inductance between windings, which is due to the energy in the magnetic flux produced by a primary winding which is not coupled to a secondary winding. One manifestation of leakage inductance is that, if the secondary winding is “shorted out”, a finite inductance is still seen at the primary winding. In effect, the leakage inductance of a transformer is electrically equivalent to placing inductors in series with one or both of the transformer windings.
The relative magnitude of the leakage inductance of a transformer can be defined as the ratio of reactive power circulating in the leakage inductance divided by the output power, at the full rated output power of the transformer. This relative leakage impedance can also be expressed as XL/R, where XL is the impedance of the leakage inductance, and R is the secondary load impedance, both viewed from the same winding. For most transformers this ratio is on the order of 2% to 10%, and is often considered a non-ideal and undesirable characteristic.
In other applications, however, the leakage inductance can be of considerable benefit. In power distribution transformers, it will limit the current under fault conditions, such as downed and shorted power lines. If the leakage impedance is 4%, for example, the fault current is limited to 25 times (1/0.04) the full rated load current, which limits the current that fuses or circuit breakers must interrupt. High leakage transformers are also used to limit or control output current in arc welders and gas tube illumination transformers.
In electronic power converters, a high leakage inductance may also be useful. In various “resonant” converters, the leakage inductance can form all or part of a resonant inductance in a circuit. Leakage inductance can also be used to aid in “soft switching” of converters, where energy stored in leakage inductance is used, for example, to bring the transistor voltage to zero before turn-on after another transistor turns off.
High Leakage Inductance Transformers
In many of these applications, however, the practical leakage inductance obtainable with conventional transformer designs is often less than that desired. Referring to FIG. 1, the prior art transformer 10 has ferromagnetic core 11, with primary 12 and secondary 13 wound on the center leg of an E-E or E-I core (so called from the shape of the core pieces or segments). In this construction, the maximum practical leakage impedance may be on the order of 5% to 10%, whereas a leakage impedance of 50% to 100% or more may be required.
Another prior art transformer construction is shown in FIG. 2, where transformer 20 comprises primary and secondary windings 22 and 23 respectively, placed on the outer legs of a so called U-U or C-C core. This construction has several benefits, including more winding cooling area and lower high frequency losses, but the leakage impedance is about half of that of FIG. 1.
A prior art transformer construction with higher leakage inductance is shown in FIG. 3, where transformer 30 has primary 32 and secondary 33 wound side-by-side on core 31. This construction may double or triple the leakage impedance over that of transformer 10 in FIG. 1, but this is still inadequate for many high leakage applications.
A prior art construction with relatively high leakage inductance is shown in FIG. 4, where transformer 40 has primary and secondary windings 42 and 43 placed on opposite legs of core 41. The leakage inductance may be further increased with the construction of FIG. 5. This construction is similar to FIG. 4, with the primary and secondary windings 52 and 53 placed on the outer legs of core 51. In this case, a “flux shunt” 54 with air gap 55 is added between windings 52 and 53, which allows leakage impedances to be three to 10 times higher than even that of FIG. 4.
The transformers of FIGS. 4 and 5 do have a major drawback in generating a large external magnetic “leakage” field, however, as illustrated in FIG. 6. Here transformer 60 again has primary and secondary windings 62 and 63 on outside legs of core 61. This is easily seen if secondary 63 is shorted out. The voltage on a winding is proportional to the rate of change of internal magnetic flux, so a shorted winding, which has essentially zero voltage, must have essentially zero ac flux in the core beneath the winding. The shorted winding 63 thus “blocks” the core flux from the leg under winding 63. This in a sense removes the winding and that part of the core from the magnetic structure, so they are shown in phantom lines in FIG. 6, and the core beneath shorted winding 63 becomes effectively a core air gap 65. The magnetic flux produced by current in primary 62 must form a closed path, so the return flux forms a large external dipole magnetic field, illustrated by flux lines 69 (a similar field develops with the transformer of FIG. 5). Secondary winding 63 need not be shorted out for this external field to develop; any load current flowing in winding 63 will cause an external field 69 proportional to the secondary current. Such external fields can cause severe electromagnetic interference (EMI) problems in higher frequency power converters, and is to be avoided.
A prior art high leakage transformer construction with reduced external field is shown in FIG. 7, where transformer 70 has primary and secondary windings 72 and 73 side-by-side, as in FIG. 3, but now flux shunts 74 are placed between the windings with air gaps 75 on each end of the flux shunts. This construction is very popular in many line frequency (50 Hz and 60 Hz) applications, including ferroresonant transformers. Drawbacks are a somewhat limited winding surface area for cooling, and higher eddy current (so called “skin and proximity effect”) losses in high frequency (HF) transformers.
An improved prior art construction is shown in FIGS. 8A and 8B. In this figure, and in FIGS. 9A and 9B, and FIGS. 11A-11B through FIGS. 21A-21B, the “A” figure is a perspective view of the transformer core and flux shunts, without the windings for clarity. The “B” figures show the location of the primary and secondary windings in a cross section through the core.
The transformer 80 of FIG. 8 is similar to that of FIG. 1, but now flux shunts 84, with air gaps 85, are placed between primary winding 82 and secondary 83. This construction increases the winding cooling area and decreases HF eddy current losses, with less of an external field that the transformer of FIG. 7.
Prior art high leakage transformers have traditionally been constructed with either laminated cores (where the orientation of the laminations is shown as 56 in FIGS. 5, and 76 in FIG. 7) or isotropic materials such as ferrite which have no “orientation”, as illustrated in FIGS. 4 and 8. These constructions cannot be directly applied to tape wound cores, where the “as wound” orientation of the tape is at right angles to that of laminations. The problem this creates is illustrated in FIG. 9, where an attempt is made to realize the transformer of FIG. 8 with a tape wound core. Here transformer 90 has a conventional tape wound core 91, with additional flux shunts 94 (and air gaps 95) cut as “bars” or “bricks” from a tape wound core. Primary 92 and secondary 93 are arranged as in FIG. 8. Magnetic flux in the flux shunts 94 now flows into transformer core 91 where they join at 98. This flux is normal to the surface of the tape in core 91, and causes large eddy current losses in core 91 at those points.
Thus high leakage transformers with tape wound cores are desired which meet two objectives: a low magnetic field external to the transformer, and principal core flux which flows from one core segment to another along the direction of the tape; i.e., principal core flux does not flow normal to the tape surface.
One potential or seeming prior art approach to meeting the second objective is shown in FIG. 10, redrawn from [2]. This core is said to be made from “ . . . rectangular shapes of amorphous metal cores . . . ”, but windings are not shown, nor is a function for the core stated. It might be that the core is intended for high leakage transformers, as it resembles the core shown in FIG. 5, with transformer core 101 and possible flux shunt 104, with the air gap 105 shown at one end of the flux shunt. However, this construction would still exhibit the same large external field as that of FIG. 5.